The fermentation of grape must is a complex process in which sugars, naturally present in grape juice, are transformed into alcohol and carbon dioxide by the action of yeasts. The fermentation process stops when the alcohol level in the wine has reached around 13 to 15 percent of the volume. In most cases, all the sugar in the grape is fermented by then and the wine is dry. Thus, all over the world, even in the warmer regions, wine tends to be dry.
Sweet wines are sweet because of three reasons: (i) the fog in the vineyard producing the noble rot and botrytised wines, (ii) the frost during the harvest resulting in ice wine or (iii) the skillfull processing of the grapes by the winemaker after the harvest in the wine cellar. Indeed, in Germany wine makers have developed extraordinary skills in arresting the fermentation or blending the dry wine with suessreserve (sterilized juice) to produce wines that display a combination of a low level of alcohol and delicious sweetness making them unique in the world.
Dorothy J. Gaiter and John Becher from the WSJ reviewed sweet American wines on October 17, 2009. A great article.
Among the 5 wines mentioned is also a sweet Riesling from the Finger Lakes, produced by the Wiemer winery, a very fine wine.
Hermann J. Wiemer Vineyard Bunch Select Late Harvest Riesling 2006 (Finger Lakes). $50. New York makes fine Riesling, both dry and sweet, and this is one good example, from a reliable name. The taste is rich with lychee, white peach and earth, but the texture is light. Clean, fresh and tangy. There was just one barrel—50 cases—of the 2006. The 2007 (about $70) is a bit more widely available.
See my blogposting of August 12, 2009: German Wine Basics: How does a Sweet German Riesling Become Sweet on more background information on the various reasons why sweet wine can be sweet.